The present invention relates to an optical unit and a process for production thereof, said optical unit being, for example, an optical filter capable of controlling the transmittance in the visible region (of wavelength 400-700 nm) and an optical filter applicable to numeral and character display units or X-Y matrix display units.
The recent development of electronics has aroused active researches on electrochromic display units and dimmers that operate electrically.
Electrochromic units are used as display units of voltage drive type, such as those for digital watches.
Since electrochromic display units (occasionally referred to as ECD hereinafter) are not of light-emitting type but of passive type to utilize reflected light or transmitted light, they offer the advantage of causing less strain to the eye after watching for a long time and of requiring a lower drive voltage and a less amount of electric power. An example of them is disclosed in Japanese Patent Laid-open No. 24879/1984. It is an ECD of liquid type, which employs as the EC material an organic viologen derivative which is reversibly colored and discolored.
The conventional ECD has the structure as explained below. FIG. 1A shows in section the laminate structure used for the ECD, and FIG. 1B shows a light beam being transmitted through the laminate at the time of operation. The laminate shown in FIG. 1A is composed of a transparent substrate 1 (of glass) with a thickness of t, patterned transparent electrodes 16a, 16b of ITO (Indium Tin Oxide) or SiO.sub.2 formed on one side of said substrate, a lead 17 (of Cr, Ni, Cu, or Au, for example) to connect said electrodes to the power source, an electrically insulating film 19 of SiO.sub.2 which separates said electrodes 16a, 16b from each other and covers said lead 17, and a light-shielding film 19 formed on the other side of said substrate. The light-shielding film 19 is composed of three layers of Cr.sub.2 O.sub.3 19a, Cr 19b, and Cr.sub.2 O.sub.3 19c. Incidentally, "C" in FIG. 1A denotes the light-shielding width.
The light-shielding film constitutes the non-display partso that it prevents a light beam from being reflected and diffused by the lead of metal (such as Cr, Ni, Cu, and Au).
The light-shielding film 19 is composed of three layers of Cr.sub.2 O.sub.3, Cr, and Cr.sub.2 O.sub.3 so as to make up their faults. The Cr layer is chemically stable and has nearly 0% transmittance for its thickness greater than 600 .ANG.; however, when used alone, it does not absorb light but reflects light. By contrast, the Cr.sub.2 O.sub.3 layer is poor in physical strength and presents difficulties in film forming; however, it has a low reflectivity (about 5%). Thus the combination of these layers differing in transmittance and refractive index makes it possible to prevent light reflection and light transmittance (by light absorption).
The laminate shown in FIG. 1B has a substance 7 deposited on the electrodes 16a, 16b. The substance 7 is viologen which is an organic compound capable of reversibly controlling the amount of transmitted light. At the time of operation, the region covered with viologen prevents a light beam incident thereon vertically or almost vertically from being transmitted. However, it permits a light beam 23 incident thereon aslant to be transmitted directly or by diffraction. This is a disadvantage of the laminate shown in FIG. 1B.
FIG. 2 shows in section an electrochromic element 40 (occasionally referred to as element 40 hereinafter) which is composed of two laminates 41, 41' arranged side by side, with their electrodes inside. (These two laminates 41, 41' are identical with that shown in FIG. 1A except that they are symmetrical to each other.) Interposed between them is an electrolyte 15 containing viologen, which is sealed with a spacer (not shown). It is assumed that the electrodes 16b, 16b' are activated and viologen 7 is deposited on the surface of the electrode 16b.
FIG. 2 shows that the light beam 25 incident on the element 40 vertically or almost vertically is shielded by the viologen 7 deposited on the surface of the electrode 16b. On the other hand, since the electrodes 16a, 16a' are not activated, the light beam 26 incident on the element 40 vertically or almost vertically passes through the element 40. However, there may be a light beam 24 incident aslant on the element 40 which passes through the element 40 although it should not.
FIG. 3 is a schematic representation showing the passage of light beams 27, 28 from a light source (not shown) to the observer's eye 42 through a lens 43 and the element 40. It is to be noted that in addition to the light beam 28 to pass through, there is the light beam 27 which should not originally pass through but reaches the eye 42. In the case of actual electrochromic display units, there will be an incident light beam which is aslant with a maximum angle (.theta.) of about 60.degree. with respect to the optical axis, and it behaves as if it had passed through the element 40 directly like the light beam 28.
It follows therefore that the structure shown in FIGS. 2 and 3 can shield the light beam parallel or almost parallel to the optical axis but cannot shield that which is aslant with a certain angle. In the case where the element is used for a dimmer, the incident light beam will be aslant with a maximum angle of about 60.degree. with respect to the optical axis. Therefore, it is necessary to increase the width C of the light shield in order to reduce the leakage light.
The width C of the light shield should be as small as possible so that the area for light transmission increases and hence the amount of transmitted light increases, thereby contributing to the image quality. The smaller the thickness t of the substrate (of glass), the easier it is to narrow the width C of the light shield. Moreover, a thinner substrate contributes to the size reduction of the optical unit. However, the possible smallest thickness t would be about 0.3 mm because of the limitation of mechanical strength.
As mentioned above, optical units such as electrochromic display units are provided with a light shielding film of three-layer structure (such as Cr.sub.2 O.sub.3 /Cr/Cr.sub.2 O.sub.3).
This light shielding film suffers the disadvantage that it cannot be formed in the usual way on the same side that on which the electrodes are formed, because the layers of Cr.sub.2 O.sub.3 and Cr are not complete insulators. Therefore, it has to be formed opposite to the electrode and lead. As the result, it permits some light beams to enter or diffract aslant with a certain angle. Such aslant or diffracted light beams (or leakage light) have to be shielded by increasing the width of the shielding film. The direct consequence is a decreased amount of light and a deteriorated image quality (due to ghosts in the image).
The light shielding film is formed by physical vapor deposition (such as vacuum evaporation and sputtering) in a vacuum, as known well. Repeating this process for the three layers takes a long time and leads to a high production cost. In addition, forming the light shielding films on both sides of the substrate needs troublesome operation for their accurate alignment. For this reason, there has been a demand for an optical unit possessing the laminate structure which suffers little or no leakage light, thereby contributing to the optical and display characteristics.